I haven't been able to reproduce this locally, but I see what's going
on, and I think we could do better.
This avoids the frame flush from nsCanvasFrame::CreateAnonymousContent,
and in general makes the anonymous content timing a lot more reasonable
(see the FlushPendingNotifications call that's getting removed).
The ServoStyleSet changes are needed now that there are multiple style
roots that can be unstyled like that.
It also simplifies the API as now it's not dependent on having a canvas
frame around (and thus doesn't need to flush layout).
Differential Revision: https://phabricator.services.mozilla.com/D250531
8ff4a65ffa